Factorisation of prismic-specific controller methods

I just put everything in a new `PrismicController` (what was in `PrismicOauthController` and the common part that was always in `ApplicationController`), and now, if you wish your `ApplicationController` actions to be able to use prismic.io, you simply need to have it extend `PrismicController`.

@dohzya, does it sound good to you?

If so, I'll generalize it to other examples we have, and then I'll make a generator for it and for the `PrismicService` and all, in the gem, if you agree it's a good idea.